1. Field
Aspects of the present disclosure relate to redox flow batteries, and in particular, to redox flow batteries having high energy density and high charge and discharge efficiency.
2. Description of the Related Art
Secondary batteries are high-efficiency energy storage systems and are used in a wide range of applications including small mobile devices and middle or large-capacity power storage devices. Also, secondary batteries are used as key components in semiconductor and liquid crystal fields, sound fields, and communication fields pertaining to portable mobile phones and notebook computers. Furthermore, recently, secondary batteries are used as power sources for hybrid vehicles.
Demand for energy storage systems to supply energy more stably and to have higher energy conversion efficiency is increasing, and recently, redox flow batteries are drawing attention as high output and highly durable secondary batteries that are particularly suitable for large-scale energy storage systems.
Unlike in other batteries, the active material of redox flow batteries is present not in a solid state but as ions in an aqueous state. The ions are oxidized and reduced at the positive electrode and the negative electrode, respectively, in order to store and generate electric energy.
That is, in a redox flow battery, the active material included in an electrode is dissolved in a solvent, that is, the active material is present in an electrolytic solution (solution). If a battery including a positive electrolytic solution and a negative electrolytic solution having different oxidation states is charged, an oxidation reaction occurs at the positive electrode and a reduction reaction occurs at the negative electrode, and the electromotive force of a battery is dependent upon the standard electrode potential level difference) (E0) of the redox couple that forms the positive electrolytic solution and the negative electrolytic solution. Meanwhile, the electrolytic solution is supplied from an electrolytic solution tank by using a pump. A redox flow battery has an advantage of a typical battery, that is, a high oxidation-reduction reaction speed at surfaces of the positive electrode and the negative electrode, and an advantage of a fuel cell, that is, high power output characteristics.
FIG. 1 is a schematic view of a conventional redox flow battery. Referring to FIG. 1, the conventional redox flow battery includes a positive electrode cell 1 and a negative electrode cell 2 separated by an ion exchange membrane 10. The positive electrode cell 1 and the negative electrode cell 2 respectively include a positive electrode 13 and a negative electrode 14. The positive electrode cell 1 is connected to a positive electrode tank 21 from which a positive electrolytic solution 11 is supplied through a pipe 41 and to which the positive electrolytic solution 11 is discharged through the pipe 41. Likewise, the negative electrode cell 2 is connected to a negative electrode tank 22 from which a negative electrolytic solution 12 is supplied through a pipe 42 and to which the negative electrolytic solution 12 is discharged through the pipe 42. The positive and negative electrolytic solutions 11 and 12, respectively, circulate through the pumps 31 and 32, and charging and discharging occurs according to valence electron change reactions at the positive electrode 13 and the negative electrode 14.
The ion exchange membrane 10 prevents mixing of the active material ion of the positive electrolytic solution 11 and the active material ion of the negative electrolytic solution 12 and allows only a charge carrier ion of a support electrolyte to pass.